Hi RoboLobo,
This is not uncommon with a wool topper. Natural fibers will compress and get firmer and how they feel will also depend on what is underneath them and the type of wool topper you are using. Wool is great for ventilation and temperature control and can also be good for “point specific” pressure point relief but it generally does best with something soft underneath it (like the latex foam you had) to provide some “give”.
When a mattress gets old and saggy … it can often become firmer. If your mattress has top layers of some type of fiber they will have compressed and become firmer and if it has soft foam in the top layers then if these have softened significantly you will often “go through” them more and feel more of the firmer layers below them in your mattress.
If your mattress has worn evenly and there are no soft spots or hills and valleys then it would be a good candidate for a topper to make it softer. If it has softened in certain spots or has deeper impressions then a topper will just “follow” any soft spots or impressions in your mattress and this could cause alignment issues … even if it solves pressure issues.
So if your mattress is suitable for a topper … or if the risk of alignment issues is not as important as pressure relief (which is often the case with fibromyalgia) … then a topper could be helpful … even if the “best” solution would be a new mattress.
There are some topper guidelines in post #2 here and the posts it links to that may be helpful but if you have an old saggy mattress and you have no other choices than a topper then a little thicker will have better odds of “somewhat” evening out the soft spots and impressions.
Post #4 here about some temporary or partial solutions to a sagging mattress may also be helpful.
Phoenix